The provision of flowlines to transport fluids to and from the above water work areas of offshore apparatus has been the subject of study for many years. The continued growth of the offshore drilling industry and their ventures into areas having more severe environmental conditions, for example, greater water depths and more severe weather conditions, such as the North Sea, has provided great impetus to find better, safer, and less costly methods of an apparatus for offshore operations.
One particular interest is the diminution of the diver assisted underwater welding operations. Diver assistance is time consuming and expensive especially at depths of 200 feet or more where movement is difficult and the amount of underwater time per dive is small. Hence any underwater operation should preferably be of minimum complexity, and preferably requires as little diver assistance as possible; otherwise, the time and expense in setting up an offshore facility quickly becomes overburdening.
Another field of interest is the reduction of capital expenditures for special equipment, for example, equipment having unusually high bending or pulling requirements, such as is used to bend pipelines in a specified manner. This equipment may be used infrequently and hence the cost per use may be relatively high.
This invention is directed to the riser portion of an offshore apparatus and in particular to a method of installing it after laying the pipeline, that is, in a laying in situation. The riser is the means by which fluids are received or discharged from the above water facilities of the apparatus. Typically, fluids may be received using a riser connection from underwater storage and/or production facilities; or fluids may be discharged from the platform through the riser to a pipeline for transport to onshore storage or production facilities. Typically, the riser is connected to a fluid processing or distribution network on the above water portion of the offshore apparatus, and at its other submersed end, to a pipeline at a connection or coupling means near the base of the apparatus on the bottom of the body of water.
Many different techniques and apparatus have been suggested in the art for coupling the riser to the pipeline. These techniques generally provide either for mechanically bending the riser pipe, which may be a continuation of the pipeline, to make an approximately 90.degree. change in direction from horizontal to vertical or for using a preformed riser section having a substantially 90.degree. bend. In the latter case, the riser section is then coupled or secured in a fluid-tight connection to the pipeline. In any instance, the requirement of bending the riser to change the orientation of fluid flow from a horizontal to a substantially vertical direction (or vice versa) is a mechanically difficult (special equipment having very large, controlled bending forces are required) and time consuming operation entailing relatively large costs.
Another approach has been suggested (Matthews, Jr., U.S. Pat. No. 3,219,119, issued Nov. 23, 1965) for providing means for coupling the riser to the pipeline using a flexible coupling or flexible joint between the rigid pipeline and the riser member. However, this arrangement involves the installation of the riser on the jacket before the pipeline is laid.
An object of the invention is therefore a method of providing a pipeline-riser installation which is relatively inexpensive and simple to install, which is reliable in operation, which permits riser installation after a pipeline is laid, and which can be installed at great depths with a minimum of diver interaction. Other objects of the invention include a method of providing a pipeline-riser installation in which large bending forces and specialized equipment are not required, and in which deformation of a rigid pipeline is avoided.